(1) Field of the Invention
The present invention relates to a color electrophotographic apparatus which is applicable to color printers, color copy machines, color facsimiles, and the like, and also to image forming units to be used in such a color electrophotographic apparatus.
(2) Related Arts
Color electrophotographic apparatuses generally form color images by superimposing yellow, magenta, cyan, and black toner images and transferring them onto a transfer member in accordance with either the transfer drum system or the sequential superimpose system.
According to the transfer drum system, different color toner images are sequentially formed on a single photosensitive member, and then transferred onto a transfer member such as paper rolled over the transfer drum by rotating the transfer drum. The relative position of these color toner images is adjusted by rotating the photosensitive member at the same speed as the transfer member and by matching the top end of each color toner.
Japanese Laid-open Patent Application No. 1-252982 shows a color image forming apparatus which employs the transfer drum system. According to this apparatus, a yellow toner image is first formed onto a rotating photosensitive drum and transferred onto a predetermined position on paper as a transfer member rolled over the outer surface of a transfer drum. Then, magenta, cyan, and black toner images are sequentially formed on the photosensitive drum and then transferred onto the predetermined position in the same manner as the yellow toner image. As a result, a color image is formed on the paper.
The transfer drum system has an advantage of managing with one photosensitive drum and a single exposure position. However, as its disadvantages, the neighboring portion of the developers and the photosensitive member tends to have very complicated construction because the positional correlation between the photosensitive member and each of the developers must be accurate.
In addition, troublesome operations for correlative positioning of the developers and the photosensitive member, or for adjusting the process conditions are necessary in maintenance, and another correlative positioning operation between the photosensitive member and each of the developers or the adjusting operation of the process conditions are necessary when the photosensitive member is replaced with another.
In contrast, according to the sequential superimpose system, an individual image forming unit is assigned to each toner color and a transfer member is conveyed on a belt or the like to pass an image forming position. As a result, each image forming unit transfers a respective toner image onto the transfer member.
Japanese Laid-open Patent Application No. 1-250970 shows a color image forming apparatus which employs the sequential superimpose system. According to this apparatus, four image forming stations each having a photosensitive member and an image exposure unit are arranged for forming color images. Paper is conveyed on a belt to pass under the four image forming stations, and as a result, four toner images are sequentially superimposed to form a color image.
The sequential superimpose system does not need a transfer drum, and accordingly there is no need to roll a transfer member over the transfer drum. In addition, the positional adjustment operation between the photosensitive drums and the developers is easy because each photosensitive drum and each developer are formed as a pair. However, providing an image forming unit for each color demands image exposure units such as laser beam systems which correspond to the number of the image forming units. This makes the construction of this portion complicated and expensive.
Furthermore, each image forming unit has its own exposure position, so that the positional correlation among the latent images formed by the image forming units greatly affects on the positional correlation among the different color toner images to be formed onto the transfer member. Therefore, the positional adjustment of the latent images formed by the image exposure units must be very accurate, and consequently, complicated construction is demanded to avoid positional disaccord as described in Japanese Laid-open Patent Application No. 1-250970.
On the other hand, another type of color electrophotographic apparatus has been developed. For example, the color electrophotographic apparatus described in Japanese Laid-open Patent Application No. 62-287264 has several electrophotographic process cartridges corresponding to the number of toner colors. According to this apparatus, each cartridge, which includes a photosensitive member and a developer having a toner, is sequentially conveyed to the image forming position. And the photosensitive member is exposed and developed for forming images which are to be transferred onto the transfer member on the transfer drum.
The cartridges are attached to a rotor frame whose rotation conveys the cartridges to the image forming position sequentially. The exposing operation for the photosensitive member is carried out by reflecting a beam emitted from outside of the rotor frame through a mirror fixed inside the rotor frame to lead the beam to a predetermined exposure position.
In this color electrophotographic apparatus, the photosensitive member and the developers can be repaired only by exchanging the cartridges, and positioning of the exposure position is very simple because the exposure position is fixed.
However, the mirror to be provided inside the rotary frame for leading a signal light to the photosensitive member demands a space for a light path to lead the signal light to the mirror.
Furthermore, in order to avoid collision between the mirror and the cartridges, the mirror is designed to be able to move away from the path for the cartridges. Or in the case where the mirror is fixed, the mirror is placed out of the path for the cartridges, making a space for the mirror in the rotary frame.
Thus the color electrophotographic apparatus has a problem that reducing the size of the rotary frame is difficult when the mirror is fixed and an additional problem that positioning of each exposure position is difficult when the mirror is movable. The difficulty in the size reducing of the rotary frame leads to the difficulty in the size reducing of the color electrophotographic apparatus.
On the other hand, in order to obtain a clear color image with few jitters, the photosensitive member must rotate at a fixed speed while an image is being formed. This is achieved by binding the drive axis of the photosensitive member with a fly wheel having a large inertia. However, such complicated construction having a fly wheel for every one of the plurality of photosensitive members makes maintenance laborious.